Plastic lenses are lightweight, highly tough and easy to be dyed. Properties particularly required for plastic lenses are: low specific gravity; high transparency; low yellowness; high refractive index and high Abbe number as optical properties; high heat resistance; high strength; and the like. A high refractive index allows a lens to be thinner, and a high Abbe number reduces the chromatic aberration of a lens.
Recently, as raw materials for plastic lenses for eyeglasses, many organic compounds having a sulfur atom have been reported. Among such compounds, polythiol compounds having a sulfur atom are, for example, reacted with an isocyanate to be used as polythiourethane resins excellent in impact resistance, or reacted with an episulfide to be used as resins having an excellent refractive index, and thus well known as useful compounds (Patent Documents 1 and 2). Among them, 4-mercaptomethyl-1,8-dimercapto-3,6-dithiaoctane is a typical compound (Patent Document 3).